Device for color measuring

ABSTRACT

A color measuring device measures the light transmitted through the material to be measured as well as the light reflected therefrom and the irradiated light and measures the color reflectivity in the various spectral ranges by dividing the reflected intensity by the difference between the irradiated and transmitted intensity. The calculation of the color reflectivity in the individual spectral ranges is thus corrected to the proper extent without the need to place color tiles behind a transparent material to be measured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of papermaking andmore specifically to a device for color measuring.

2. Description of the Related Art

EP 0 240 610 B1 shows a device for measuring humidity and density of aweb-shaped material at which coherent electromagnetic radiation withdifferent frequencies is directed to the material and the reflected andtransmitted radiation is evaluated. Herewith the emitted and irradiatedradiation is not sensed.

If the material under test as e.g. paper is not completely opaque, thencolor measuring raises some difficulties. Under laboratory conditions itis possible to fold the material under test repeatedly in order toachieve a complete opacity, whereas this is not possible with respect tomoved single-layer webs on paper machines. To this respect it istherefor prior art to produce an artificial background. To this endcolored tiles, the color of which is matched to the material under testto be measured, are brought behind the material under test. Thedisadvantage of this solution consists in that already with smalldistances between the material under test and the color tile an errorarises in the color measuring. Moreover, the variety of papers which areproduced on a machine is often very large, and, therefore, it is notalways possible to back it by a color tile with the right color.

Departing from this prior art, it is therefor the object of the presentinvention to devise a color measuring device which, independent of theopacity and without the requirement of backing the material under test,allows an accurate color measurement.

BRIEF SUMMARY OF THE INVENTION

The solution of this object is achieved by an improvement to a devicefor the color measuring of materials under test, in particular paperwebs. The device includes a light source arranged to generate light in aplurality of spectral ranges and means for splitting the lightirradiated by the light source into a first and a second beam of light,the first beam directed onto the material under test. The device furtherincludes a first measuring apparatus for receiving and generatingsignals representative of the intensity of the light reflected off ofthe material under test for each spectral range received, and a secondmeasuring apparatus for receiving and generating signals representativeof the intensity of the light of the second beam for each spectral rangereceived and a color computer for receiving and processing the signalsfrom the first and second measuring apparatus. The improvement comprisesa third measuring apparatus arranged to receive light from the firstbeam that is transmitted through the material under test and to generatesignals representative of the light received therethrough for eachspectral range received, whereby the color computer receives andprocesses the signals from the third measuring apparatus by subtractingthe signals received from the third measuring apparatus from the signalsreceived from the second measuring apparatus, thereby forming adifference signal for each spectral range. The color reflectivity isdetermined by the color computer by dividing the difference signal foreach spectral range from the signals received from the first measuringapparatus for the associated spectral range.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

With respect to the figures of the attached drawing in the following theunderlying problem of the invention and its solution shall be furtherdescribed with respect to an embodiment whereby:

FIG. 1 shows the spectral reflectivity of a material under test as afunction of the wavelength of the light with the thickness of the layeras a parameter;

FIG. 2 shows the spectral reflectivity of a material under test as afunction of the wavelength with the distance of its backing as aparameter;

FIG. 3 shows a device for color measuring according to the prior art;and

FIG. 4 shows the improved device for color measuring of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

From FIG. 1 it is discernible that over the spectral wavelength rangebeing of interest at the color measuring, the reflectivity which isevaluated at color measuring shows strongly different values as afunction of the thickness of the layer of the material under test to bemeasured.

FIG. 2 shows in the same way that at backing of the material under testwith a selected color tile the reflectivity shows strongly differentvalues as a function how large the distance is between the materialunder test and the color tile.

FIG. 3 shows a known device for color measuring which is subject of theabove-mentioned disadvantages.

A material under test 10 preferably of paper extends between the lowerportion 12 and the upper portion 14 of a measuring head which istravelling across the material under test 10 or is arranged stationarilywith respect to it, respectively. Within the upper portion 14 colortiles 16 having different colors are arranged on an octagon which bymeans of a background controller 18 may be brought into a position wherethey are backing the material under test 10.

Within the lower portion 12 xenon flashlight ring lamp 20 is arranged,the light of which on one hand via a beam splitter 22 is reflected tothe material under test 10 and on the other hand through the beamsplitter 22 is impinging on a reference measuring location 24. The lightreflected from the material under test 10 is sensed at a measuringlocation 26. Since the xenon flashlight ring lamp 20 has a spectrumranging up to the UV range and the UV portion within the light projectedonto the material under test 10 is producing light within the visiblerange in the event where the material under test contains a brightenersubstance which is sensed at the measuring location 26 and is tamperingthe measuring, a further xenon flashlight ring lamp 20' is arrangedbelow and symmetrical to the beam splitter 22, and the beam splitter 22is provided with the behaviour of a UV filter. Both of the ring lamps20, 20' are alternately operated so that light is alternately receivedat the real measuring location 26 as well as at the reference measuringlocation 24 with the light not containing the UV portion or the lightproduced by the UV portion, respectively, whereby accuracy of themeasuring is improved.

The light radiation received at the measuring location 26 and at thereference measuring location 24 each via fiber bundles 28, 28' and afiber optics 30, 30' is fed to optical filters 23, 23', behind whichphotodiodes 34, 34' are arranged, which sense the signals within thedifferent spectral ranges. Those signals are fed to a computer 38 via asignal processing means 36 with said computer calculating the colorreflectivity within the different spectral ranges from the relationbetween the reflected intensity and the irradiated intensity. Thecomputer controls the selection of the according color tile 16 via thebackground controller 18. Additionally, it controls the alternateswitching-on of the ring lamps 20, 20'.

The inventive device according to FIG. 4 is distinguished over the knowndevice according to FIG. 3 by the provision of an additional measuringlocation 40 within the upper portion 14 of the measuring carriage forthe reception of light being transmitted through the material undertest, whereat the measured signals again via a fiber bundle 28" and afiber optics 30" are fed to optical filters 32", behind which photodiodes 34" are arranged. The signals of the photodiodes 34" if necessaryafter an according signal conditioning are fed to the computer 38 whichcalculates the color reflectivity within the different spectral rangesaccording to the following relationship: ##EQU1## By this calculationthe color reflectivity is corrected in the right way. Herewith it is tonote that the correction for each wavelength is individually done.

The present invention has been described with particular reference tothe preferred embodiments thereof. It will be obvious that variouschanges and modifications can be made therein without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. An improved device for the color measuring ofmaterials under test, in particular of paper webs, the device measuringcolor reflectivity and including a light source arranged to generatelight in a plurality of spectral ranges, means for splitting the lightirradiated by the light source into a first and a second beam of light,said first beam of light directed onto the materials under test and afirst measuring apparatus for receiving and generating signalsrepresentative of the intensity of the light reflected by the materialsunder test for each spectral range received and a second measuringapparatus for receiving and generating signals representative of theintensity of the light of said second beam of light for each spectralrange received and an evaluation device for receiving and processing thesignals from said first and said second measuring apparatus to determinethe color reflectivity of the materials under test, the improvementcomprising:a third measuring apparatus arranged to receive the lightfrom said first beam of light that passes through the materials undertest and to generate signals representative of the intensity of lightreceived for each spectral range, whereby the evaluation device receivesthe signals from said third measuring apparatus and in the determinationof the color reflectivity for each spectral range of said plurality ofspectral ranges the signal from said third measuring apparatus issubtracted from the signal from said second measuring apparatus.
 2. Animproved device for color measuring as claimed in claim 1, wherein saidevaluation device is a computer and said computer determines said colorreflectivity by dividing said signals from said first measuringapparatus by said signals from said second measuring apparatus wherebyin said improved device said computer further receives said signals fromsaid third measuring apparatus and said signals from said thirdmeasuring apparatus are first subtracted from said signals from saidsecond measuring apparatus developing a difference signal and the colorreflectivity is thereby determined by dividing said signals from saidfirst measuring apparatus by said difference signal.
 3. An improveddevice for color measuring as claimed in claim 2, wherein the devicefurther includes a measuring head and said measuring head includes afirst portion and a second portion with said materials under testlocated therebetween, said first portion containing said light sourceand said first and said second measuring apparatus therein whereby insaid improvement, said third measuring apparatus is contained in saidsecond portion.
 4. An improved device for color measuring as claimed inclaim 3, wherein said first measuring apparatus includes measuringlocation optically connected to a fiber optic network by a fiber opticbundle and said fiber optic network is optically connected to aplurality of photodiodes, said photodiodes electrically connected tosaid computer and optically connected to said optical filters, andresponsive to said measuring location receiving said light reflectedfrom said materials under test, said light received is transmitted viasaid fiber optic bundle to said fiber optic network and said opticalfilters to said photodiodes whereby said photodiodes generate andtransmit to said computer electrical signals representative of theintensity of light received for each spectral range of said plurality ofspectral ranges.
 5. An improved device for color measuring as claimed inclaim 3, wherein said second measuring apparatus includes a measuringlocation optically connected to a fiber optic network by a fiber opticbundle and said fiber optic network is optically connected to aplurality of photodiodes, said photodiodes electrically connected tosaid computer and optically connected to said optical filters, andresponsive to said measuring location receiving said second beam oflight, said light received is transmitted via said fiber optic bundle tosaid fiber optic network and said optical filters to said photodiodes,whereby said photodiodes generate and transmit to said computerelectrical signals representative of the intensity of light received foreach spectral range of said plurality of spectral ranges of said secondbeam of light irradiated by said light source.
 6. An improved device forcolor measuring as claimed in claim 3, wherein said third measuringapparatus includes a measuring location optically connected to a fiberoptic network by a fiber optic bundle and said fiber optic network isoptically connected to a plurality of photodiodes, said photodiodeselectrically connected to said computer and optically connected to saidoptical filters, and responsive to said measuring location receivingsaid light passed through said materials under test, said light receivedis transmitted via said fiber optic bundle to said fiber optic networkand said optical filters to said photodiodes, whereby said photodiodesgenerate and transmit to said computer electrical signals representativeof the intensity of light received for each spectral range of saidplurality of spectral ranges.